The present invention relates to separation processes for purifying multicomponent fluids and separating such a fluid into its constituent components, and in particular to separation processes for the recovery of rare gases, such as krypton and xenon, from air and other such multicomponent fluids.
Krypton and xenon exist in the atmosphere in very small quantities, but are finding increasing application in industry and other uses. Xenon is being used for medical applications, including as an anesthetic, and in space applications as a propellant. Krypton is used in lighting applications and as an insulating gas in high efficiency windows. The typical source of krypton and xenon is atmospheric air, which contains approximately 1.14 ppm krypton and 0.087 ppm xenon.
In recovering krypton and xenon from air, it is common practice to distill liquid oxygen, which has been removed from the bottom of a lower-pressure column of a cryogenic air separation plant, to obtain an enriched stream containing krypton and xenon. Hydrocarbons within the air also are carried into the liquid oxygen and hence are concentrated during the subsequent concentrating process of krypton and xenon. The enrichment of the liquid hydrocarbons may create a flammable mixture. To avoid this potential hazard, removal of hydrocarbons by adsorption or replacement of oxygen by inert gases (such as nitrogen) is conventionally carried out, or the enrichment of krypton and xenon within their oxygen carrier is progressed to a state wherein the fluid does not enter the “region of flammability.” The concentration of krypton and/or xenon in the stream is therefore dictated by the ability to suppress the potential for oxidation.
U.S. Pat. No. 5,122,173 (Agrawal, et al.) discloses a process wherein krypton and xenon are recovered from a liquid oxygen stream from a cryogenic air distillation unit. Most of the heavier hydrocarbons are removed in an adsorption step prior to distillation. However, more than 80% of methane and lighter hydrocarbons, such as ethane, ethylene, and acetylene, is not removed in an adsorption step. The distillation process simultaneously concentrates krypton and xenon while rejecting a substantial quantity of the methane present in the feed stream. Therefore, the methane removal primarily takes place through a distillation process.
U.S. Pat. No. 4,421,536 (Mori, et al.) discloses a process wherein krypton and xenon are recovered from a liquid oxygen stream from a cryogenic air distillation unit. A large portion of the methane present in the cryogenic air distillation unit feed stream is removed as a purge gas from a distillation column. The remaining hydrocarbons are removed through a catalytic oxidation and an adsorption process. Therefore, there is an equilibrium separation step (distillation) that precedes hydrocarbon removal by catalytic oxidation.
U.S. Pat. No. 5,993,612 (Rostaing, et al.) discloses a process for the purification of a feed gas using a hollow dielectric tube. A feed gas is passed through a hollow dielectric tube wherein it is converted, through the use of an electric field, into an atmospheric pressure plasma which is not in local thermodynamic equilibrium. On leaving the dielectric tube the gas passes over a reactive material for eliminating the reactive compounds from the gas to be purified. The impurities in the feed gas are removed through the use of a reactive material, wherein a hollow dielectric tube is used to aid in this reaction. The process typically requires the addition of oxygen to the feed gas in order to aid in the reaction between the impurities and the reactive material.
The reactions between oxygen and hydrocarbons, especially methane, in a corona discharge unit are discussed in “Production of Organic Oxygenates in the Partial Oxidation of Methane in a Silent Electric Discharge Reactor,” by Larkin, et al., American Chemical Society (2000). A methane stream containing oxygen is fed to the corona discharge unit. This gas is then exposed to a silent electric discharge converting up to 59% of the methane to full and partial oxidation products. The process emphasizes the production of organic liquid oxygenates, a partial oxidation product.
A corona discharge unit is an example of a hydrocarbon removal system that can be employed prior to any distillation steps.
It is desired to have an improved process for recovering rare gases, such as krypton and/or xenon, from a multicomponent fluid.
It is further desired to have an improved process for recovering rare gases, such as krypton and/or xenon, from a multicomponent fluid which overcomes the difficulties and disadvantages of the prior art to provide better results.
It is still further desired to have an improved process for recovering rare gases, such as krypton and/or xenon, from a multicomponent fluid, such as purge from an air separation unit, which overcomes the difficulties and disadvantages of the prior art to provide better results by reducing the number of equilibrium separation steps, such as distillation or single-stage flash.